This invention relates to a printer having an optical printing head for the line-by-line image forming of text and design information. The printing head is of the type that has a plurality of light emitting devices (assemblies) each having a plurality of selectively energizable light emitting elements arranged in a row. The printing head further includes an optical device for reproducing, in one line, the light emanating from the light emitting elements on a light sensitive surface of a carrier such as a drum.
In data processing systems rapid printers are needed to transform the electric input signals into a visible representation readily recognizable as a printout. For this purpose, printers with optical printing heads have been successfully used. Thus, German Offenlegungsschrift (Application Published without Examination) No. 2,557,254 discloses an optical writing apparatus working with a light sensitive record carrier, photographic paper or intermediate record carrier and having an electrically controlled optical component with image dot elements which block or transmit the light flow. German Offenlegungsschrift No. 2,812,206 discloses an optical printer having a magnetically controlled optical component, based on an integrated light modulating matrix. This system does not need a light beam to be deflected or an electrostatic charging for the printing step proper. Such an integrated light modulating matrix, disclosed, for example, in German Offenlegungsschrift No. 2,606,596, is based on the light modulation by magneto-optical memory layers such as almandite layers. The layers are formed of a uniform arrangement of light switching elements which are switched solely electronically via vapor-deposited conductor and resistor strips. In case of high integrating density, line-shaped light switching components having over 1,000 elements may be constructed. The light switching elements are arranged between a permanent light source and a light sensitive record carrier and are controlled by a character generator in such a manner that the light continuously emitted by the light source can be blocked or transmitted at predetermined image grid dots as required. For forming a single straight image line on the light sensitive surface, there are required a large number of light switching elements arranged side-by-side in one row and transversely to the direction of advance (feed) of the record carrier. Between the light switching elements and the record carrier there is provided an optical system for adapting the image to the printing surface of the record carrier.
Further, German Offenlegungsschrift No. 3,031,295 discloses an optical printer wherein the switchable light switching elements are formed of solid state light emitting diodes. These light emitting elements are combined block-wise into light emitting assemblies which are arranged alternatingly in two rows parallel to the imaging lines; with each row there is associated a separate row of optical image forming system comprising a Selfoc lens. Such a two-row arrangement is necessary for producing a continuous line on the record carrier, since the light emitting elements are shorter than their carrier body and the image forming optical system can reproduce only in a 1:1 ratio.
In prior art structures the light emitting elements are designed as narrow strips having a dimension of 20 micron in a direction parallel to the feed of the record carrier and 90 micron in the transverse direction. By maintaining the period of illumination for one feed increment of approximately 80 micron of the intermediate carrier, there is obtained an illuminated spot of approximately 100 micron. This means that in case of a relatively high operating speed of a printer, the light emitting elements must operate with a high output of light emission which may lead to a shortening of service life particularly because of increased heat removal problems.
For generating a uniform, throughgoing light line on the light sensitive surface of a record carrier, a uniform spacing of the serially arranged light emitting elements is required. The small distance of 10 micron necessary for a high resolution requires high manufacturing precision and renders assembly and adjustment difficult.